1. Field of the Invention
The invention is related to the field of disk systems, and in particular, to a system that controls a servo timing mark detection window.
2. Statement of the Problem
FIG. 1 depicts a current disk drive that includes a disk device 100 and a disk drive processing system 130. Those skilled in the art are aware that numerous conventional aspects of the disk drive are not shown for clarity. The center of the disks 101-102 are each attached to a perpendicular spindle 107. A spin motor 108 spins the spindle 107 and the disks 101-102 at a constant rate. The heads 103-106 read information that is stored on the disks 101-102. The heads 103-106 pass signals containing this information to a pre-amp 120. The pre-amp 120 passes only one of the signals from a selected head to the read channel 121 where the signal is processed into a format suitable for a processor 131. The processed signal (or signals) is then passed to the processor 131 in the disk drive processing system 130. The processor 131 executes firmware stored in the memory 134 to control the operation of the disk drive. This includes positioning the heads 103-106 relative to the disks 101-102 and exchanging user information with the disks 101-102. The processor 131 and memory 134 could be a single integrated circuit or a group of integrated circuits.
The disks 101-102 each contain circular tracks that store both servo data and user data. The servo data includes position information that allows the disk drive processing system 130 to identify locations on the disks 101-102. The servo data is uniformly spaced around the tracks of the disk, so that the heads 103-106 regularly encounter servo data as the disks 101-102 spin. Thus, the servo data is read at regularly spaced time intervals based on the constant spinning rate and uniform placement around the disk.
The servo data contains servo timing marks (STMs). STMs are used to locate and identify the corresponding servo data. An example of an STM may be no magnetic transitions on the disk for 350 nanoseconds followed by one magnetic transition followed by no magnetic transitions for another 350 nanoseconds. Each STM must be detected during a period of time known as a STM detection window that is generated by the disk drive processing system 130. The STM detection window prevents false STM detection in cases where user data has similar characteristics to the STM. False STM detection can cause a catastrophic failure of the disk drive. The processor 131 may be used to detect the STM during the STM detection window, or additional circuitry (hardware) may be used for STM detection.
The processor 131 opens and closes the STM detection window by using the hardware timers 132-133. FIG. 2 illustrates this process. The processor 131 starts the hardware timer 132 when the first STM is detected in the signal. When the hardware timer 132 expires, the STM detection window opens for detection of the second STM. The false STM located before the STM detection window opens is ignored. The second STM is used by the disk drive processing system 130 since it is detected while the STM detection window is open. The processor 131 starts the hardware timer 133 at the opening of the STM detection window, and the STM detection window closes when the hardware timer 133 expires. The false STM located after the STM detection window closes is ignored.
Head switch operations add complexity to STM detection. During a head switch operation, the pre-amplifier 120 switches the signal that it passes to the disk drive processing system 130. For example, the pre-amplifier 120 might pass a signal from the head 103 before the head switch and pass a signal from the head 106 after the head switch. It should be appreciated that during a head switch operation, the processor 131 must detect successive STMs in two different signals.
FIG. 3 depicts the head switch problem. The first STM may come from the head 103 signal, and after a head switch, the second STM may come from the head 106 signal. Physical mis-alignment between the heads 103 and 106 can cause timing misalignment between the first and second STMs as shown in FIG. 3. In this case, the hardware timers 132-133 may fail to keep the STM detection window open until the second STM is detected. On FIG. 3, the legitimate second STM would be ignored since it is detected after the STM detection window closes. A failure to detect the STM requires a recovery procedure before normal operation can resume.
Mis-alignment is caused by thermal warping in mechanical assemblies or disk slippage from physical shock or handling. Removable media drives pose additional alignment problems. The servo data on removable disks is written by equipment that has different mechanical alignment than the actual disk drive used to read and write to the removable disks.
The current solution to this problem is to simultaneously reduce the length of the hardware timer 132 and increase the length of the hardware timer 133 during a head switch operation. This lengthens the STM detection window during a head switch operation to account for some mis-alignment. Unfortunately, the maximum length of the increase for hardware timer 133 must be estimated and fixed during the design of the hardware in the disk drive processing system 130. If this design does not provide the proper amount of additional time, STM detection might fail during head switch operations where severe misalignment is present.
The normal duration of an STM detection window is 2-6 microseconds. This is typically increased to 20 microseconds during a head switch operation. Since the STM detection window is centered around the expected moment of STM detection, the STM detection window has 10 microseconds before and after the expected detection time point. Unfortunately, STM shifts of 50 microseconds or more can occur due to severe misalignment during head switch operations, especially with removable media drives. Redesigning the disk drive processing system 130 to increase the length of hardware timer 133 would be costly and impractical.